Such a device is for example seen in DE 10 2004 053 929 A1, where a mixture of granulate and cooling water is supplied to a centrifuge dryer which is arranged adjacent to a cooling arrangement. In the method of granulate drying shown therein, the adherent surface water is centrifuged off, together with plastic dust which also clings to the surface. At the same time, evaporative cooling takes place by the drying of the surface moisture of the granulate in an air stream. A screen-like water separator is arranged between the drying arrangement and the centrifuge dryer.
The centrifuge dryer comprises a cylindrical housing and a cylindrical screen fixedly arranged therein. Arranged within the screen is a motor driven rotor that can be driven at high revolutions. The rotor comprises paddle-like arms that accelerate the wet granulate and the water that still adheres to it. Due to the design of the arms, the granulate is propelled from the lower inlet upward and is dewatered at the same time. The water is centrifuged off through the screen.
Residual surface moisture remains on the granulate and is removed by evaporation in the centrifuge dryer. This cooling-down drying is supported by an air stream, which is aspirated from the top into the housing and is exhausted by a blower at approximately half the height of the housing.
Instead of the paddle-like arms, other known devices of the relevant art comprise baffles affixed to the rotor and arranged at an angle to the vertical axis, causing a vertical conveyance of the granulate within the centrifugal dryer from the inlet opening on the bottom upward to the outlet opening.
The disadvantage of these known devices is that the wet granulate is always supplied to the centrifuge dryer radially, whereby the granulate is caught frontally by the paddle-like arms or baffles and first propelled against the fixed screen from where it rebounds and is again captured by the paddle-like arms or baffles. This process is repeated until the granulate reaches the outlet opening and passes through it. The frontal capture of the granulate exposes it to higher mechanical stress in the inlet area. This results in greater amounts of plastic dust, which settles in the immediate area of the inlet opening, aided in particular by air stream eddies. The removal of the dust by means of cleaning fluids introduced into the centrifuge dryer space by nozzles is more difficult in this area than in the remaining inner space of the centrifuge dryer.
This can go so far that, especially for the drying of sensitive granules, in particular of composite granulates, the known centrifuge dryers cannot be used at all for the dewatering and drying of these granulates because too high a proportion of the granulate would be damaged by the known centrifuge dryer.
Due to the frontal capture of the granulate, the fixed screen and the paddle-like arms or baffles in the region near the inlet opening are exposed to much higher wear than in the regions above. Since the outlet opening is also arranged radially, there is also higher wear in the immediate vicinity of the outlet opening, especially at the frontal edge.
A further substantial disadvantage is the overall design of the device which makes it difficult to maintain. The centrifuge dryer has to be cleaned completely after every change of granulate color or composition. Any remnants that are not removed completely before the next load of plastic granulate passes through the centrifuge dryer contaminate the granulate and result in substantial quality flaws.
For this purpose, several nozzles for the cleaning fluid are commonly provided inside the centrifuge dryer. Wherever the deposit of plastic dust is particularly heavy, for example, as mentioned earlier, in the vicinity of the inlet opening, spraying of the cleaning fluid is not always sufficient, or the cleaning is only satisfactory with disproportionately great amounts of the cleaning fluid. However, since the expense of preparing the cleaning fluid is proportionate to the quantity, it is in most cases more economical to open the device, loosen the remaining residue by hand and to rinse it out. In known devices, this requires disproportionately high disassembly and reassembly expenditures.